1. Field of the Invention
This invention relates to a focus detection method for detecting the camera focus (hereinafter referred to as focus) from contrast signal values of the optical image of an object and the light intensity distribution of the image using light receiving elements.
2. Description of the Prior Art
The optical image of an object focused by the photographing lens has a characteristic that the difference in the intensity between bright and dark parts of an image, i.e., the contrast thereof, is maximum when the image is exactly focused. The focus thus can be automatically detected by detecting the maximum contrast.
Among the hitherto proposed methods of focus detection, there is one called a contrast system. FIG. 1 shows this system. As is shown, the system includes light receiving element rows 3 and 4 which are optically spaced apart a fixed distance in a direction parallel to the optical axis of a photographing lens 1. The system includes a light splitter 2 for amplitude splitting the optical image from the photographing lens 1. The light splitter 2 includes a semi-transparent mirror 2a and a reflecting mirror 2b, these mirrors being disposed to re-direct the incident optical image to the respective light receiving elements 3 and 4. Contrast signal value of the detected optical image are calculated using a predetermined evaluation equation to detect which one of the two light receiving elements rows 3 and 4 is closer to the image forming plane of the photographing lens 1.
FIG. 2 shows the contrast signal value which is given as a function of the distance l.sub.e covered by the lens. Curves C.sub.1 and C.sub.2 show contrast signal values of the optical images detected by the first and second light receiving element rows 3 and 4. Since the optical image from the lens is equally split by the light splitter 2 into two images, the curves C.sub.1 and C.sub.2 are identical in shape but are spaced apart in the distance axis direction by an amount corresponding to the optical distance l.sub.o between the first and second light receiving element rows 3 and 4.
The point F of intersection of the two curves C.sub.1 and C.sub.2 corresponds to a point at an equal distance from the two light receiving element rows 3 and 4, i.e., the mid point between the light receiving element rows 3 and 4. This point usually constitutes a focal point, which is contained in a focal plane Q on which the optical image of an object P is focused. When the distance l.sub.e covered by the lens is changed in the neighborhood of the point F,
that is, when it is changed to l.sub.e =l'.sub.e, a difference AC is provided in the contrast signal value C between the optical images from the first and second light receiving element rows 3 and 4. In this case, the image forming plane is determined to be closer to the first light receiving element row 3 of the higher contrast signal value C. The photographing lens 1 is thus moved for focus adjustment until the contrast signal values C of both the light receiving element rows 3 and 4 are equal.
Similar focus adjustment is also made when the image forming plane Q is determined to be closer to the second light receiving element row 4.
In regions apart from the intersection point F, i.e., on the left hand side of the neighborhood of point N.sub.1 and on the right hand side of the neighborhood of point N.sub.2 in FIG. 2, the object image is extremely out of focus, and the contrast signal values C of both the rows are very small and substantially equal. For example, when l.sub.e =l".sub.e, the difference between both the contrast signal values C is very small. If an error in the whole detecting apparatus is substantial, therefore, it is difficult in this case to determine which one of the light receiving element rows the image forming plane Q is closer to.
As has been shown, with the prior art focus detection method based on the contrast system, the image forming plane cannot be detected if it is greatly out of focus although it can be well detected and brought to focus if it is in the neighborhood of the focual point.